Hole-transport properties of a low-band gap alternating polyfluorene

The bulk transport properties of positive carriers in thin films of a low band-gap conjugated polymer, called APFO-Green5, have been investigated in the ac regime. The frequency-dependent impedance of an ITO/PEDOT:PSS/APFO-Green5/Al structure (where ITO is indium tin oxide and PEDOT:PSS is poly(3,4-ethylenedioxythiophene)/polystyrene sulphonic acid) was measured as a function of the dc applied bias. The capacitance response at low frequency gave indication of a combination of trapping and double-injection effects, while in the intermediate-high frequency range was determined by the transit time of injected holes. Hole mobility in APFO-Green5 thin films exhibited a Frenkel-like dependence on the applied electric field, with a field-dependent coefficient of around $8×10−3(Vcm−1)−1/2$⁠. A hole mobility close to $2×10−5cm2V−1s−1$ was achieved at the field of $3.5×105Vcm−1$⁠, in excellent agreement with that already reported by using a different bulk investigation technique.